When ore material is sintered, it is mixed with a fine-particulate, carbon-containing solid and deposited on a sintering belt on which it is transported to a discharge end, and while it is advancing on the sintering belt at least some of the solid material is combusted. Alternatively, the starting material is formed into pellets or briquettes and then sintered. Combustion air is supplied. During sintering, the feedstock undergoes a low-temperature pyrolysis process, and at least some of it is subjected to a combustion process that has the effect of agglomerating the feedstock in larger lumps, that is to say it is sintered. Exhaust gases are given off in considerable quantities by the feedstock in the combustion and low-temperature pyrolysis process, to which the combustion air is added, and besides CO2, possibly O2, H2O, and or N2 they contain a wide variety of harmful components. These particularly include nitrogen oxides (NOx), SO2, HCl, dioxins, furans, dust, and sublimatable or condensable residues from the low-temperature pyrolysis process, heavy hydrocarbons and/or heavy metals.
Air pollution prevention studies have revealed that the exhaust gases from sintering belts for example are responsible for a very significant portion of the total pollutants that are created in the steelmaking process. For example, in terms of dioxins and furans, fractions exceeding 90% of the pollutants were detected in the corresponding emissions in the iron and steelmaking process. Due to the extraordinarily large quantities of exhaust gases given off by sintering belt systems, until now it has not been possible to clean the gases satisfactorily without effort of the order that would render the overall cost of steelmaking more expensive. In particular, because of the different proportions of harmful components in the sintering belt exhaust gas, together with their widely varying compositions depending on the feedstock, besides their widely differing reactions and the diversity of the available cleaning methods, it has been necessary to link several cleaning steps in a consecutive series.
Thus for example, entrainment processes have been suggested with downstream filtering of entrained particles and further downstream catalytic oxidation to reduce dioxins. The catalyst was severely degraded in such processes, the catalyst surface being covered in a deposit of organic hydrocarbons (Final report 50 441-5/217 “Reduction of dioxin emissions from sintering systems” commissioned by the German Federal Environmental Agency, December 2002). Another exhaust gas cleaning method for sintering belt systems was suggested in WO 01/17663, to the effect that the sintering exhaust gas was cleaned in an entrainment cleaning stage followed by an adsorption cleaning stage, and in which ground, high-grade activated carbon with a relatively small particle size is added to the exhaust gas in the entrainment stage, so that an entrainment cloud is formed. The finely particulate adsorption medium reacts in the entrainment phase with some of the harmful components that are to be removed from the sintering belt exhaust gas. However, in the post-reaction stage after the entrainment stage the flue dust was not precipitated onto a fabric filter or electrofilter, but onto the input side of a counterflow moving bed reactor, where the flue dust was precipitated on the particles of the moving bed particle material, that is on the surface or in the interstitial volumes thereof. The sintering belt gas then passed through the particle layer of the counterflow moving bed reactor, consisting for example of activated carbon, so that the sintering belt gas was precleaned in an entrainment phase before it underwent cleaning by adsorption. The entrainment cleaning process upstream of the moving bed reactor requires the use of a second particulate cleaning agent, which however does not prevent the disadvantageous degradation of the catalyst in the moving bed.
Especially when it is most important to remove the NOx from the sintering exhaust gas, other polluting components such as SO2 and HCl have proven particularly inconvenient when the NOx is to be removed from the exhaust gas by a catalyst, because they and other pollutants in the sintering gas are “catalyst poisons” when it comes to NOx removal.